1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a technology of charging and exposure.
2. Description of the Related Art
A typical electrophotographic image forming apparatus includes an image forming station configured to perform toner development with regard to an electrostatic latent image formed by exposing a charged photosensitive drum to form a toner image on the photosensitive drum, and a transfer portion configured to electrostatically transfer the toner image formed on the photosensitive drum to an image-receiving member. An image forming apparatus configured to form a full-color image includes a plurality of image forming stations, and, by transferring in sequence respective single-color toner images formed in the respective image forming stations to an image-receiving member such as an intermediate transfer member or a recording material so as to be superimposed on one another, a full-color image may be formed.
In the transfer portion, by a voltage applied to the transfer member, the toner image is electrostatically transferred from the surface of the photosensitive drum to the image-receiving member. In this case, transfer of charge from the transfer member to the photosensitive drum may cause fluctuations in a surface potential of the photosensitive drum. The extent of transfer of charge depends on whether the recording material as the image-receiving member is located in the transfer portion or not, that is, whether the recording material exists between the photosensitive drum and the transfer member or not. In particular, when the recording material is not introduced to the transfer portion, charge is liable to be transferred from the transfer member to the photosensitive drum, and thus, fluctuations in surface potential of the photosensitive drum are liable to occur. Therefore, after the recording material passes through the transfer portion, a surface potential difference may be caused on the photosensitive drum.
Further, when a toner image is formed on an image-receiving member such as a recording material or an intermediate transfer member, transfer of charge is suppressed by the toner. In the case of a full-color image forming apparatus, when a toner image is transferred in a downstream image forming station in a state in which a toner image is already formed on the image-receiving member in an upstream image forming station, the surface potential of the photosensitive drum is less liable to fluctuate at a location of the image-receiving member at which a toner image is already formed. Therefore, a surface potential difference may be caused on the photosensitive drum after the image-receiving member passes through the transfer portion between a location at which a toner image is already formed on the image-receiving member at the time of the transfer and a location at which a toner image is not formed as yet on the image-receiving member at the time of the transfer. Such a surface potential difference (transfer memory) on the photosensitive drum after the image-receiving member passes through the transfer portion may remain after the photosensitive drum is recharged by a charging unit, which is a cause of uneven density of a halftone image.
Transfer memory can be erased by increasing the amount of charge when recharging is performed by the charging unit. Japanese Patent Application Laid-Open No. 2008-8991 discloses means for forming a potential necessary for image formation by, after transfer memory is erased by charging a photosensitive drum to a potential which is higher than a potential necessary for image formation, exposing the surface of the photosensitive drum to light to a small extent.
However, with regard to the means disclosed in Japanese Patent Application Laid-Open No. 2008-8991, the photosensitive drum is charged to a potential which is higher than that necessary for image formation, and thus, electric discharge between the photosensitive drum and the charging unit may become larger and deterioration of the photosensitive drum may be accelerated. Further, a potential necessary for image formation is formed by exposing the photosensitive drum to light to a small extent, and thus, the exposure may also accelerate the deterioration of the photosensitive drum.